1. Field of the Invention
The present invention relates to a process for fabricating highly uniform ceramic electroactive transducers. More particularly, the present invention relates to a process for fabricating hollow ceramic electroactive transducers, which are essentially spherical in shape.
2. Description of the Prior Art
A large number of medical procedures which require catheters are performed in the United States each year. Catheters are, typically, plastic tubes having a diameter of few millimeters. The uses of such catheters include, intravenous drug delivery, therapeutic device delivery, and other types of invasive procedures, such as, guiding balloon angioplasties of the leg, guiding catheters inside the heart to ablate incorrectly functioning cardiac tissue, and guiding catheters within the uterus to inject fluid into the fallopian tubes to test for tubal blockage.
Catheters must be guided to the proper location in the subject. One method of guiding catheters is to employ ultrasonic imaging. In doing so, an ultrasonic transducer is mounted on the catheter and a receiver is placed on the subject. When an ultrasound signal from the catheter mounted transducer strikes the external receiver, the signal is converted into an electronic marker corresponding to the precise location of the catheter in the ultrasound image of the subject.
At present, catheters are guided with real time X-ray imaging known as fluoroscopy. The use of ultrasound imaging has not found wide acceptance despite the advantages ultrasound has over X-ray technology.
A major encumbrance to using ultrasound imaging in guiding catheters is orientation-dependent ultrasound visibility of catheters known in the art. The orientation-dependent visibility, which plagues this technology, results from the wavelength of the ultrasound being several times smaller than the catheter is wide. The result is a catheter that becomes an ultrasound reflector, in addition to being an ultrasound transducer.
This produces an ultrasound visibility that is highly dependent on the orientation of the mounted transducer. X-ray fluoroscopy is not hindered by this phenomenon.
In one approach to eliminate orientation dependant ultrasonic visibility, an omnidirectional ultrasonic transducer is directly mounted on one end of the catheter. This approach solves the problem of signal orientation dependence on catheter orientation. To be suitable for this use, the ultrasonic transducer must be omnidirectional, possess low signal loss, have a high sensitivity, and preferably be uniform in construction and inexpensive to produce. Use of such an omnidirectional ultrasonic transducer mounted on a catheter would thus allow for replacement of the hazardous X-ray imaging commonly used in guiding catheters. The end result would be a more cost efficient, less dangerous procedure benefiting doctors, hospitals, and their patients.
Many different types of ultrasonic transducers such as thickness mode, polymer based, solid core and hollow core transducers are known in the art.
Thickness-mode ultrasonic transducers possess low signal loss, high sensitivity, and are inexpensive to produce. However, they are highly directional and as such, do not meet the omnidirectional requirement.
Polymer based piezoelectric materials can be easily fabricated into different shapes, including an omnidirectional geometry such as a sphere. However, polymer based piezoelectric materials possess high signal losses and low electromechanical coupling coefficients, which render polymer based transducers unsuitable for this use.
Solid core ceramic spherical transducers are omnidirectional. However, low sensitivity makes them less than suitable.
In contrast to the above transducers, hollow sphere ceramic electroactive transducers have the required omnidirectionality, low signal loss and high sensitivity required. In addition, they can be easily matched to electronic systems. Thus, hollow sphere ceramic electroactive transducers promise a natural solution to the problems associated with guiding catheters using ultrasound-imaging technology. However, given the current state of the art, it is extremely difficult to fabricate uniform hollow sphere ceramic electroactive transducers.
Presently, hollow sphere ceramic electroactive transducers can be produced by machining and grinding bulk ceramic into hemispheres. An electrode is then formed on the inner surface of the hemispheres, followed by bonding of two such hemispheres together. This approach is labor intensive, high cost, and low in productivity.
U.S. Pat. No. 4,917,857 to Jaeckel et al. is directed to a process for the production of metallic or ceramic hollow spheres to make a reticulate structure. In this process, foamed polymer cores are coated with a suspension containing metal or ceramic particles in a bed reactor. The polymer core is later pyrolyzed to obtain a metal or ceramic hollow sphere. A hollow sphere ceramic electroactive transducer can be prepared using hollow ceramic spheres obtained from this process by first opening a small hole in the sintered sphere, normally by polishing. Next, an inner electrode is deposited on the inner surface of the hollow sphere through the hole. An outer electrode is then deposited on the outer surface of the sphere. The ceramic is then poled under an electric field. However, this method, and the transducers produced by this method have low green density. In addition, the ceramic shells contain many pores due to the forming process that hinder subsequent sintering of the green ceramic body. The pores become defects in the surface of the sintered sphere, leading to mechanical fractures and decreased sensitivity when the spheres are processed into hollow sphere ceramic electroactive transducers.
Additionally, the hole created is mechanically weak. This weakness limits the hydrostatic pressure tolerance of the transducers produced in this fashion. When used in underwater applications, the depth capability of these transducers is drastically reduced.
Thus, as discussed above, the crude fabrication techniques present in the art do not allow for hollow ceramic electroactive transducers to be produced uniformly, in commercially significant quantities, at a cost low enough to make this technology appealing.
The present invention includes a process for fabricating a ceramic electroactive transducer of a predetermined shape. The process includes the steps of: (a) providing a suitably shaped core having an outer surface; (b) attaching a first conductor to the outer surface of the core; (c) coating an inner conductive electrode on the outer surface of the core such that the inner conductive electrode is in electrical communication with the first conductor; (d) coating a ceramic layer onto the inner electrode; thereafter (e) sintering the ceramic layer; (f) coating an outer electrode onto the sintered ceramic layer to produce an outer electrode that is not in electrical communication with the first conductor; and (g) poling the sintered ceramic layer across the inner electrode and the outer electrode to produce a ceramic electroactive transducer.
The present invention further includes a process for fabricating a ceramic electroactive transducer of a predetermined shape which is multilayered. The process includes the steps of: (a) providing a suitably shaped core having an outer surface; (b) attaching a first conductor to the outer surface of the core; (c) coating a first inner conductive electrode on the outer surface of the core such that the first inner conductive electrode is in electrical communication with the first conductor; (d) coating a first ceramic layer having an outer surface on the first inner electrode; thereafter (e) attaching a second conductor to the outer surface of the first ceramic layer; (f) coating a second inner conductive electrode on the outer surface of the first ceramic layer such that the second inner conductive electrode is in electrical communication with the second conductor without being in electrical communication with the first conductor; (g) coating a second ceramic layer having an outer surface on the second inner electrode; thereafter (h) sintering the first and the second ceramic layers; (i) coating an outer electrode on the second sintered ceramic layer to produce an outer electrode that is not in electrical communication with the first conductor or with the second conductor; and (j) poling the first and the second sintered ceramic layers across the first inner electrode, the second inner electrode, and the outer electrode.
The present invention still further includes a process for fabricating a plurality of ceramic electroactive transducers, which have a predetermined spatial relationship of a predetermined shape. This process includes the steps of: (a) providing a plurality of suitably shaped cores, each having an outer surface, arranged in the predetermined spatial relationship; (b) attaching one or more first conductors to the cores; (c) coating an inner conductive electrode on each of the outer surfaces of the cores such that the inner conductive electrodes are in electrical communication with at least one of the first conductors; (d) coating a ceramic layer on each of the inner electrodes; thereafter (e) sintering the ceramic layers; (f) coating one or more outer electrodes on each of the sintered ceramic layers to produce a plurality of outer electrodes not in electrical communication with the first conductor; and (g) poling the sintered ceramic layers across the inner electrodes and the plurality of the outer electrodes.
The present invention provides for hollow ceramic electroactive transducers by using a core formed from a thermally decomposable material, and further including the step of heating the layered ceramic under conditions sufficient to remove the core prior to the sintering step.
In addition, the process further provides for encapsulating the ceramic electroactive transducer in a non-conductive material after coating with the outer electrode.
The present invention provides a simple, low cost process to produce ceramic electroactive transducers that have a uniform inner electrode. The present invention also can produce large quantities of, for example, spherical ceramic electroactive transducers having a diameter ranging from about several tenths of a millimeter to several meters. In addition, the present invention eliminates the need for machining and polishing, is simple and economical, and substantially improves the uniformity and reproducibility of the end product.